It's a MOBA (Multiplayer Online Battle Arena) that pits different characters, chosen by the players, against each other. There are objectives, roles and a very deep meta-game that goes on while playing. Map control and awareness will guarantee most matches, but higher levels of play are found. It's very strategy and timing based. I personally believe that it and other MOBAs will make you a better player and FPS player in general. You become much more aware of timing, map control and audio/visual cues.

Edit: Please note that this description barely scratches the surface and probably doesn't do it and other MOBAs any justice.